This invention generally relates to antennas and, more particularly, to axial mode helical antennas which are used for VHF or UHF bandwidths.
An antenna is a device that functions to radiate electromagnetic energy (transmitter) and/or intercept electromagnetic radiation (receiver). Antennas may be characterized by a number of features, including their directivity and gain. Directivity relates to how well an antenna transmits or receives a signal relative to a particular direction. A nondirectional antenna transmits or receives signals equally well in all directions. In contrast, a directional antenna transmits or receives signals better in a particular direction. By positioning a directional receiver such that its directivity is aligned in the direction of incoming signals from a transmitter, one is able to improve the reception of these signals.
Antenna gain is a concept closely related to directivity. Gain is simply a comparison of the transmitting or receiving ability, in a particular direction, of an antenna in question and some reference antenna. Gain may be expressed as a voltage ratio or in decibels. Antennas have increased gain when they have sharper directivity. Enhanced directivity in a receiver reduces the effect of unwanted signals from other directions due to effects such as reflection from surrounding objects.
In a helical antenna, the directivity is along the axis of the helix conductor. One may increase the gain along this direction by increasing the number of turns of the helix and/or positioning a reflector behind the helix conductor.
In prior art helical antennas, the diameter of the reflector is usually greater than a free space wavelength of the antenna since directivity decreases as the diameter of the reflector decreases. In general, because of this decrease in directivity, antennas of this type are not used when the diameter of the reflector is less than about 0.8.lambda., where .lambda. is the free space wavelength.
Small antennas are desirable in many applications. It is further desirable to decrease antenna size while increasing the directivity. However, small antennas of the prior art have suffered from a loss in directivity associated with a small reflector. The present design will permit the construction of small antennas without the loss of directivity due to the decreased reflector size in small antennas of the prior art.